CMOSFET, which consists n-type and p-type metal-oxide-semiconductor field effect transistors, is the essential component in the integrated circuit. As the size of the devices shrinks, it is required to replace the n-type polysilicon gate with its p-type conterpart in order to create surface channel on the p-type MOSFET. The replacement can reduce both the short channel effect and hot carrier effect. In practice, BF.sub.2.sup.+ is implanted to form p-type polysilicon gate and source/drain shallow junction. Unfortunately, fluorine speeds up the diffusion of boron ions in the gate oxide. Consequently, the boron ions reach the silicon substrate and the electric properties and the reliability of the device are deteriorated. For examples, the breakdown field and the charge to breakdown can get smaller and the charge trapping rate of the gate oxide can be increased.
According to the report in IEEE Electron Device Lett. Volume EdL-14, page 179, by G. W. Yoon etc., nitrogen in the gate oxide can stop the penetration of boron ions and is therefore used in the growth of gate oxide. In addition, according to the report in IEEE Electron Device Lett. Volume 15, page 109, U.S. patent application Ser. No. 179,016 and Ser. No. 336,970 by Z. J. Ma etc., NH.sub.3 is used during the anneal process of the gate oxide. The electric properties of the gate oxide by the process deteriorate while the reliability of the gate oxide by the process also deteriorates due to the incorporation of the hydrogen. Moreover, according to the report in IEEE Trans. Electron Devices Volume 42, page 1503 by C. Y. Lin, stacked p-type polysilicon (or silicon) gate structure increases the sheet resistance which is harmful to the operation of high speed devices.